The pre-ACT budget announcement, however, will show Labor falling $8 million short of its 2012 pre-election commitment to spend $28 million on the upgrade.

In 2013, Labor committed $2.2 million to prepare the site and undertake initial upgrades and has come under some community pressure to continue the promised work.

Ms Burch said the significant investment of $19.8 million would transform the school, with work expected to be completed by 2017.

"Students at Belconnen High School will have the opportunity to learn in new and upgraded spaces designed to facilitate the best educational outcomes," Ms Burch said.

"This commitment demonstrates the government's dedication to providing the best opportunity to learn in the best environments. Works will include major refurbishment of student learning and teaching spaces, an external outdoor learning area and a new school administration area.

She noted the funding package would also help the government meet future enrolment needs in the area and provided environmental sustainability improvements.

Detailed design work will commence immediately, with construction and refurbishment work expected to commence in February 2016 and be completed by late 2017.

Yet the funding shortfall has been criticised by the Australian Education Union for short-changing the suburb.

ACT branch secretary Glenn Fowler said: "Labor could be rightly proud of their election promises in 2012, but their delivery has been consistently underwhelming."

He said ACT schools had seen only a fraction of $70 million promised for upgrades of older schools.

"It's interesting that now Belconnen High School has been deemed not worthy of the money it was promised. Once again it appears that the case for the downscaling of commitments hasn't been made to key stakeholders," Mr Fowler said.

A centrepiece of Labor's pre-election education commitments in 2012, the $28 million was to cover a total overhaul of the main and north blocks and the construction of a new two-storey building in the existing internal courtyard.

Ms Burch said: "Initial design works and studies have been taking place for some years on this project … Studies and consultations on the needs of the school and its students have determined that Belconnen High requires a major refurbishment of student learning and teaching spaces, an external outdoor learning area, and a new school administration area."

"We're pleased to be able to deliver these upgrades more efficiently than was originally estimated," Ms Burch said of the new funding total.

But Kim Fischer, a local parent who will be seeking pre-selection for Ginninderra with the ALP for the upcoming 2016 ACT election, said she was surprised and disappointed by the cut.

Ms Fischer worked briefly in Ms Burch's office as a fill-in adviser earlier in the year but said she felt morally bound to speak out against the cuts as she campaigned to bring better government services to Belconnen.

"The redevelopment of Belconnen High School was a centrepiece election promise in 2012. It was meant to be finished by now. Not only has it barely started, it is having its funding cut by more than $8 million.

"Local parents will be rightly disappointed that this school is being short-changed by nearly one-third of the promised $28 million," Ms Fischer said.